1. Field of the Invention
The present invention relates to an infrared sensor, and relates, for example, to an uncooled infrared sensor with a high-sensitivity and a wide dynamic range.
2. Background Art
Infrared imaging has an advantage of being able to image during day and night, and has higher permeability in smoke and fog than imaging by visible light. Furthermore, infrared imaging can obtain temperature information of an object. Therefore, infrared imaging has a wide range of application to security fields and the like, as a monitoring camera and a fire detecting camera.
The largest drawback of a quantum type infrared solid-state imaging device as a conventional main device is that the infrared solid-state imaging device requires a cooling mechanism to carry out a low-temperature operation. In recent years, a “non-cooled infrared solid-state imaging device” which does not require this cooling mechanism is being developed progressively. The uncooled or thermal type infrared solid-state imaging device converts an incident infrared ray having a wavelength of about 10 μm into heat using an absorption structure. A thermoelectric converter converts a change in the temperature of a heat sensing section generated by weak heat into an electric signal. The thermal type infrared solid-state imaging device reads this electric signal to obtain infrared image information. For example, an infrared sensor using a silicon pn junction that converts a temperature change into a voltage change based on a constant forward current is reported (see, Tomohiro Ishikawa, et al., Proc. SPIE Vol. 3698, p. 556, 1999).
The uncooled infrared sensor with a silicon pn-junction reads a signal as follows. When a forward current is passed to a pn-junction that is disposed in each pixel and thermally isolated from a substrate, the infrared sensor reads a voltage corresponding to an operation point that changes based on the temperature of the pn-junction. In this system, a silicon on insulator (SOI) substrate is used for a semiconductor substrate. The sensor can be manufactured in only a silicon LSI process, and this manufacturing method is suitable for mass production. A function of a row selection is realized by using rectification characteristics of the silicon pn-junction as a thermoelectric converter. Therefore, a pixel structure is extremely simple.
Noise equivalent temperature difference (NETD) which represents temperature resolution of the infrared sensor is one of the indicators showing the performance of the infrared sensor.
It is important for the infrared sensor to decrease the NETD, that is, to decrease a temperature difference that becomes equivalent to noise. To meet this, it is necessary to increase the sensitivity as a signal and to decrease noise. In order to decrease noise, it is effective to increase a bias current as a measure for decreasing current shot noise. Based on this method, a differential resistivity of the pn-junction can be decreased.
However, along with the increase in the bias current, the following problems arise.
A bias current that flows through pixels disposed in parallel in the same row concentrates in the row selection line, which causes a voltage drop. Due to this voltage, drop, a voltage distribution is caused inside the row selection line, and an output signal is inclined to a horizontal direction, thereby causing shading.
In order to avoid this shading, there is proposed a method of taking a difference between a voltage in a reference voltage line and the output signal, by providing the reference voltage line having the same voltage distribution as that in the row selection line, as disclosed in Japanese Patent Application Laid-Open No. 2003-222555.
However, an uncooled infrared solid-state imaging device disclosed in Japanese Patent Application Laid-Open No. 2003-222555 has the following problems. When a bias current is passed to the pn-junction, Joule heat is generated in the pn-junction. Due to this Joule heat, self heating occurs in the pn-junction. For example, when a change in the temperature of an object is 1 K, a change in the temperature of the pixel pn-junction is only a few mK at most. However, the self heating becomes a few K to several dozens K, and buries a signal component. As a result, a large part of a dynamic range of a readout circuit is consumed, and an actual dynamic range is suppressed.
According to the Patent Literature 1, an exclusive reference voltage line (a bias line) is necessary, and a constant current source and a differential amplifier circuit need to be provided in each column. Therefore, the readout circuit becomes complex. Consequently, a sophisticated shading correction method is required.